Rumack: Fragmentation slows down SD Card performance!
Striker: Surely you can't be serious.
Rumack: I am serious... and don't call me Shirley.
The word is slowly [but surely :-)] getting out that fragmentation affects NAND Flash storage - specifically free space fragmentation. The SD Association website is a great resource for edification and development resources. On their page describing the SD Card Speed Classes, they even discuss fragmentation.
Fragmentation and Speed
The memory of a card is divided into minimum memory units. The host writes data onto memory units where no data is already stored. As available memory becomes divided into smaller units through normal use, this leads to an increase in non-linear, or fragmented storage. The amount of fragmentation can reduce write speeds so higher SD card speeds help compensate for fragmentation.
There are several methods to address this issue. One is, as mentioned, buy better performing storage and hope your requirements never exceed the cards ability to deliver on your needs (though fragmentation will still limit the storage's peak performance). However, a better approach is to fix the root issue, and there are two ways to do that.
1. Defragment the free space (e.g. HyperFast, Diskeeper)
2. Copy all the data off the SD Card and reformat the card
The best approach is likely to be determined by the device in which the SD Card is used. If it's from your digital camera, option 2 is probably pretty easy to undertake. If pulling all the data off the card is not feasible, optimize it.
The only other question then might be "how often do I run optimization?". I did a blog post on that, including some performance tests, a few years ago, Read that Here or Here.
Lastly, I thought I'd include a recent personal success from an IT professional who happens to also use Diskeeper at work:
"Happy new year!! I made an interesting test over the holidays using Diskeeper 2010. I was updating my GPS with new maps and discovered that my Garmin unit uses Fat32 file system so I figured I would run your software on it just for fun and see the results. I was able to defragment a large part of the files and it more than doubled the speed of the unit. The images render faster with less stutter, routes are recalculated almost instantly now and it finds points of interest much faster. I did the same test on 2 other GPS`s and got the same results." Regards, Carlo